Many boxes do not have much of a lip. That is one of the characteristics I look for when building a cage with this design.
It's very likely you won't be able to use screws at the "handle end" of the box. As long as most the box has enough room that should be sufficient. That's also why I used a high-quality polyurethane caulk to further hold the box to the frame.
I'm sure you've read this from me before, but I recommend build these from a piece of plywood rather than making your own face frame from clamped pieces of wood.
The hole you cut out does not need to be square. It could be a circle if you wanted it. All that matters is that the two strips of wood that the acrylic door rests between are parallel to each other.
With a frame like the 50 gallon cage I built a greater degree of accuracy is required.
You might be able to get a scrap piece of nice plywood for little $$. Get your confidence up before you start with a full sheet for future projects.
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Right now all i'm really looking to do is try my hand at building one and get a feel on how things go together.
I'm very much a hands on person I learn alot better if I do something then have it explained to me. So I'd like to try to build one even if it comes out horrible. I'll have learned something from it and have made my mistake on a that cage I
didn't really need then on one i'm trying to build because there an need for it. hope that makes sense.
What I want out of the enclosure is something that's light enough to be easly moved by one person (which this design looks like it is),and looks good from the front and possibly one side (which it does and can be made to), can be stacked (can be made to).
I know there are ways to make this less expensive. There are always way to make things less expensive. You have to find ways this hobby is costly. Saved some money yesterday by picking a 20 & 10 gal out of the neighbor's trash.
On another note. I was looking at store totes last night when I was at walmart. I can't believe that you can screw thru the small lip that they have with out cracking them. The lips just look too small.