I was thinking of cutting a hole in the bottom and overlaying a piece of plexiglass over the entire bottom of the cage and attaching the flex watt to the underside of the plexiglass.

I have done this but have not used plexiglas. Instead I would recommend 1/4" expanded PVC or maybe even FRP, both of which are known to be better at transmitting heat than acrylic.
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Currently keeping:

6.10 Gonyosoma oxycephalum (Javan, mixed colors)

0.0.4 Rhynchophis boulengeri

1.1 Philodryas baroni

1.1 Lampropeltis triangulum multistriata

0.0.1 Rhamphiophis rubropunctatus

0.0.2 Morelia viridis (Aru & Merauke locale types)

1.0 Rhodesian Ridgeback

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