There's a lot of door and window gasket available in hardware stores...If I had to make a gasket quick I'd look at that...
Personally I prefer silicone* in the small hand tubes that don't require a caulk gun....you get much more control with the small ones and can lay down the minimal bead needed to seal it easily. (The kitchen and bathroom version of this)
https://www.homedepot.com/p/GE-Advan...-3TG/100179996
Roundy CRTs usually have a solid half inch of unphosphored bulb edge and the cabinet mask usually cuts off a bit of the phosphor all around too... even when I'm not fully concentrating I don't think I've done worse than 2/3 of the unphosphored edge and unless you install it in the cabinet WAY off center you'll never see it even if you press your face to the glass and look at strange angles. Generally try to achieve a fairly even ~1/4" bead between the two.
*If you get the glass spotless you'll never have to open it to clean it again...gaskets may leak or their material degrade but silicone if kept out of the elements should last my lifetime. The one of the first CRTs I did that was caulked almost a decade ago still looks perfect.
You can open the silicone up if you have to...2 of the first 3 tubes I did had Electronic defects (one the cathodes died, the other was gassy) and a few years later I harvested the safety glass from those 2 with an exacto knife to reuse on good CRTs that had worse condition safety glass.