lol i'm just reading and catching up on DIY threads today!
i think your measurements for the shelf height is off. those Iris tubs are probably 6" tall w/ the lid on. besides i think a 1/4" gap is way too big. i would go 1/8 at most and would rather add holes to a tub, rather than having the gap to wide. i would find the actual Iris tub and measure it in person for spot on accuracy.
i think 6" heat tape is perfect. Freedom Breeders and ARS Racks use 5in heat panels (heat tape run along metal panels of the same width). i use the rule of 1/3 heat source for tanks and PVC enclosures, etc. whereas racks are more streamlined.
if u really wanna cut a groove for the heat tape, use a router w/ a jig that gives u precise depth and width for the heat tape. or use table saw w/ dado blades. your dad would know. if u wanna snake 1 long piece of heat tape through each shelf, u will need to add cuts to the left and/or right side of each shelf.
happy building!