Edited version
2003 Model GT 5000
Hope spraying round-up will be easier this summer.
Bolted a 2x4 to each side of the rear hitch and extends to outside edge of rear tires.
The 2x4's under the tank slide on top of the previous two 2x4's and one bolt runs through both vertically. A wing nut holds bolt secure. To remove tank from tractor just remove this one bolt and pull backward.
Wish hydro had foot control.
And also need someway to secure spray wand and hose.

You may find it necessary to hang some weight off the front bumper to counter the added weight extended at the rear (when you are using spray apparatus). without some weight on the front, the steer tires may want to slip in tight turns or even become airborne under certain conditions.
As to a way of "securing" the spray wand and hose........I presume you mean some kind of holder or scabbard to rest the wand in so it is safe from slipping away but still easy to reach when needed................For that I suggest you fabricate something using a suitable length of PVC pipe attached to (or bungeed to) a handy location on the tractor where it can be managed from the operator seated position.
I also suggest that perhaps you might want to abandon the 2 X 4 material for the actual working model of your creation.
You will be better served if you use the wooden version as a prototype model for someone to fabricate a steel version from. There are going to be significant dynamic stress loads on the main supporting pieces that might very well cause the Pine 2 X 4s to split at the bolt locations (especially at the hitch area).
If wood is to be your preferred material you might at least consider switching to Oak for the main supporting members (at the hitch and under the plywood panel).

I agree would rather have a metal frame but I got rid of my welder and plan on upgrading mower (Zero turn) within 2 years probably. At a later date I may get some 1" & 1/2" angle iron make all the cuts, pre drill holes where needed and then take to someone for welding.
Wanted a smaller tank but not available. For spot spraying 5 gal mix will suffice. Have test drove it with 5 gal water up steep incline without incident.
Securing the spray wand with some type of scabbard was also my intention. The extra length of hose is cumbersome and drags on occasion.