To this point our 15x8x30 studio is used for still photography.

We now plan to work with some videographers to record interviews and while this a great, very quiet space with respect to ambient sound, the sheetrock walls/ceiling and laminate floor with the accompanying hollow/echo-y sound do not work for recording the audio.

Looking for a "low-tech" way (short of completely redesigning structures!) to make the space suitable for doing the recording?

Carpet the floor and put up some sound absorbing wall panels. You could probably hang carpet on the walls to save money but the panels will look more professional. No doubt there must be a lot of diy solutions out there.

The other issue you need to deal with is a way to quietly cool the room with hot lights.

There's a great audio magazine called Sound on Sound - been around for nearly 30 years, and they do a studio SOS feature where the experts go to people's home studios to sort problems - many like this. Their usual quick test and fix before spending money is duvets - they get a boom microphone stand, raise it fully, and set the boom horizontal = a T shape, then they drape the duvet over it and listen. The duvet is great at HF, OK in the mid range and lesser effective at the bass end. If this cures the problem, then they source proper sound treatment - traps, diffusers, reflectors, absorbers - but these cost lots - so from time to time, the studio owner just can't afford proper treatment. They have in the past, knocked up timber frames, sufficiently deep to put a duvet in, then covered these with fabric, so they look nicer. If you can't afford the usual solution - then framed duvets are crude, but will deaden things down nicely. Bass, of course goes straight through - but your problem is with voice frequencies - 300 to 3K or there about.

Grab a few duvets and dot them around and see how the sound changes. If they cure it, then maybe just posh them up with a frame - maybe even a frame on castors? For voice, even heavy drapes work fairly well - wool serge or velvet types are pretty good with the higher frequencies. The usual cause is the hard surfaces being parallel - so duvet-ing just two walls can be pretty effective. With the hard floor - you could even build a duvet frame and hang it above - from chain or similar. Chicken wire mesh is than needed to stop the duvet sagging, but again, you can pretty it up.

To this point our 15x8x30 studio is used for still photography.

We now plan to work with some videographers to record interviews and while this a great, very quiet space with respect to ambient sound, the sheetrock walls/ceiling and laminate floor with the accompanying hollow/echo-y sound do not work for recording the audio.

The reason you have "small room sound" is because it's a small room. That is, the time between the direct sound from the source arriving at the microphone and the time the first reflections from walls/floor/ceiling arrives at the microphone is short. This is the root cause of "small room sound". Bigger rooms sound bigger because the time of arrival for direct and reflected sounds is a bigger difference.

So... what can you do about it? You can try to control the reflections. The two main controls are absorption and diffusion. Too much absorption sounds dead. Too much diffusion sounds smeary. How to get the right balance is way beyond the scope of a simple web posting.

There are books out there on how to design and treat a studio. Find one and read up. What you want to do is doable, but you'll have to customize it to your space, budget, and timeline.

Carpeting the floor is the first step, even low-pile office type carpeting will help. Then what ever else you can get to put close to the the walls (moving/sound blankets are pretty low cost.. Practicals.. furniture, draperies and such will help as well. If it's in the budget, quite a few specially panels are available.

Do you have a budget in mind? How are your DIY skills? By low ech do you mean cheap, or easy to implement.

The room you describe is a box with parallel walls and ceiling, great for resonances/standing waves.

https://www.acoustimac.com/acoustic-panels/acoustic-panels-dmd
is one of many on-line sources with some helper apps.

Good recommendations on acoustic treatments in this thread. It sounds like you have good isolation. Controlling reflections is indeed the next step.

Except... as still photographers moving into video, you may not know that camera mounted microphones, such as the teeny shotgun mics sold for dSLRs, are pretty much useless for interview sound. They can be pretty handy for b-roll sound, or for exterior person-in-the-street pop-up interviews.

If an on-camera mic is your standard approach to interview sound, it’s time to get going on lavalier (lav, clip-on) mics for interviews. Getting the mic close to the source of the sound is 90% of the battle in any acoustic environment!

If you’ve not yet done this, your room acoustics may well turn out to be better than you thought!

Recommend also *not* using a shotgun for echo-y indoor locations.