steve, here are the measurments from a different canopy i built. i really think this is a much easier route to take. it might cost about 10 dolalrs more in the end, but will be well worth it. it's much easier to put together, and will be much sturdier as well.
this canopy was also made for a 55 aga
i'll attach some photos too. bearing in mind, this canopy was made simply to house a tank with versa hoods, so it's not as tall as the measurments i'll give you. since yo uare going with mh's you will want to make this canopy using at least 1x10 or 1x12 lumber. it was also made to be larger than the tank, so that it would accept a 75 tank in the future. so the sides are longer than the measurement i'm giving you.
this is also an open back canopy, which is what you want.
if you are REALLY bad with diy, you may want to look in the shelving section instead of the lumber section and get 3/4" x10 or 3/4"x12 then you will havea lot less sanding to do once you slap it all together.
1x2 12 3/4" long (2 of these) side supports
1x2 48 1/4" long (2 of these) top front/back supports
1x10 or 1x12 12 2/3" long (2 of these) sides
1x10 or 1x12 49 3/4" long (1 of these) front
1/4" plywood 49 3/4"x13 1/2" (1 of these) top
you can put trim around the front and sides if you like, or not.
being as this is an mh canopy, i would also get:
1x8 24" long (2 of these) for doors
what yo uwould do then is cut open two door holes in the front 1x2 and then cover them with these two smaller 1x8's OR do one really long door in the front. the options are endless and simply based on how you want the tank to look.
and of course, you'll need four brass hinges.
you'll notice from the pics that i did not stain or polyeurathane the inside of this canopy. big mistake. moisture is a mother! i highly recommend that you DO polyeurathane the inside of this.
if you've never used it before, they sell some stuff at hd and lowes, that is by minwax, it's a wipe on poly. just pour it on a rag, and wipe it on. wait for it to dry and do it again. since it;s inside and not visible, you dont really need to worry about sanding in between coats. i would do 3-4 coats, with no sanding.
if you have ANY questions, jsut ask, feel free to pm me too.
i'm here for ya!