My husband built our Frankencage out of 2 dog crates like we have for our Great Dane, making it 2.5 ft by 8 ft & we just love it. The crates came with "roll up" doors on one side & "swing open" doors on the end. Now that the crates are attached, we aren't using 2 of the doors. My husband offered to add a loft -- using the extra doors as its base. Or with proper support, we could easily zip tie in some C&C grids as the base. Given the configuration of the crates, their doors & how it's all put together, we have a cage that opens up nicely & has really great access for cleaning & such.
So it's definitely possible to make a great piggy cage using a dog crate.
Plus, with a little creativity, you should be able to add a shelf without any big problem. Dog crates do vary quite a lot, though. So you will want to make sure that what you create ends up being a completely safe, roomy & comfortable home for your piggies!
On that note, I will say that the Frankencage was kind of expensive -- much more so than a C & C cage of equal size. All other things being equal, have you considered getting/building a C & C cage? So many people have them & love them. And it sounds like they are a great way to save a few bucks, so more $$ for extra fleece, hideys or your piggy-vet fund. Welcome & Best of Luck!
PS. Bonus Thoughts: The "giant crate" option is plenty tall enough for a 2nd floor & since a crate is self-contained, if you need a closed cage, the "lid" is already in place. Here's a quick pic for ideas. (That's my girl Emmy checking out the very first cage liner I made for them.) I'm a sewing nOOb, so please just ignore that part. haha
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