It is basic 1/4 MDF inside of the wood frame with finishing nails. On the inside and what I should probably include in the build are 90degree flat angle brackets on the top corner by handle and opposite bottom corner by hinge.
built the doors at a different location from home where I added these later ( hence no pictures ). I'll take some when I get home :p.
I did this method because I wasn't about to drop $120 on a new set of router bits when I have a table saw that can pull off a similar effect. Just gotta reinforce the corners.
You could have done it without a router at all. You were half way there. You did the groove part. Cutting the tongue (or stub tenon same thing in this case) was even easier. I just did 8 doors for a shaker style cabinet this way.
The router bits would only be if you wanted to profile the inside of the frame and form a cope and stick joint. A straight profile is easy-peasy.
Another idea to strengthen it would be to have used a spline joint on the miter. Again, all on the table saw.
The stand looks good. Looking again at it though it seems as if you are relying on the fastener to hold the weight of the tank.
Hope you take this in the helpful manner I mean it in. Hard to tell the tone of someone typing on a forum so I will say outright it's meant in only a constructive manner.