Are you looking at the little bitty shelf, or the big bottom one? (I'm going to assume bottom)
That particular conversion would be very easy. Drill a few holes in the ceiling of the lower portion for ventilation. Drill a few holes in the bottom so you can get ventilation for your heat element. Place your UTH on the bottom and foil tape it down with your T-stat probe on/under it (May want to run the T-stat probe under the cabinet up through one of your UTH vent holes). Silicone down some vinyl tile on the floor (and up the walls probably). Once that's dry add substrate and snake. Assuming there's already a door you plan on using that is secure, you're done.
This is what I mean by adding vinyl tile btw...
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