When I was DIY'ing my viv, I used polycrylic to seal some of miscellaneous bits. I haven't used polyurethane to compare, but in researching around supposedly polyurethane was more likely to break down under long-term exposure to UVB/heat, but nothing I ever saw was a particularly detailed or rigorous explanation for how long it takes or how dangerous it is, so I don't know that it actually makes a difference vs. polycrylic. Honestly as long as it's not going to sit directly under your basking spot, I feel like it won't matter anyway.
I will say from personal experience that the polycrylic was much thinner than I had anticipated, and even with two full coats, the wood I was covering still had rough patches and the general wood texture, so if you're going to seal anything close to porous, be prepared for a lot of coats. Might take a while, but afterward just give it some time to ventilate, sniff test to make sure you can't smell it, and then it should be good to go in your beardie's viv.