I've got a pretty good eye for level, straight, parallel, etc, from 20+ years in construction. I've spotted a 1/4 inch divergence in two, supposedly parallel, wingwalls on a front porch entry, and they were twenty two feet tall.
You trestle tower looks ok to me, jig or no jig, at least from the angle shown.
That said, jigs are really easy to make. A scrap of luan plywood, a sharp pencil and straightedge, a few scraps of small wood blocks, and some CA and you can finish a jig in 15 minutes or less. Sometimes, if a jig might see hard use, I'll double up on the adhesive, an array of little dots of CA for instant service, and a similar, but offset array of Elmers for longevity.
Sometimes I use a little scotch tape so that glued members don't stick to the jig, but even wood glue joints on a wood jig don't stick to problem level if you carefully move them after twenty seconds or so of clamptime.
Your rubber bands do a good job, but here's a tip I found out by accident that may serve when a rubber band won't.
From interior trim and cabinetry work, I have a big set of Irwin Quick Grip clamps, four each at 12", 18", 24" and 36", and as a joke, I boight a pair of the tiny little 6 inchers just to yank my boss's chain. He wasn't amused, but it turns out I use those little clamps on just about every wood railroading project I do. As with the big Irwin's, there's a limit to how much pressure you can get before they begin to slip, WAY less than screw clamps, but for modeling projects, they yield more than enough pressure for glue joints in wood and were well worth the $15.
I can't tell you how many times I've done that.
I've even done worse.
More than once, I've been in the middle of a project, all limbs engaged, wished i had a certain tool, muddled on through in spite of being improperly equipped, gone out and purchased the wished for tool, and then discovered I had one all along and now have two.
It's rough getting old, but there is one consolation.
One of these day's I'll get so senile I have to hire a pretty young nurse to tie my shoes.
Life could be worse.
Wish I had room for more bridges, so I could do some steel ones. Yours looks great. Maybe when the layout here is further along, I'll cook up some...display stands...in the form of nice bridges supporting favored trains or locomotives. Better than wishing and not doing.