Why would you need another tool? Use SH5's gui editor. You'll have to copy paste the page into a new file, load it up in the editor, and off you go. You'll only be able to see the interface at 1024x768 resolution but if you're careful and read my "how to" tutorials (just search for threads started by me) you should be ok.
The big problem is not editing the visuals of the interface, it's adding functionality. The periscope screen is pretty much closed, you can't do anything more with it. I've said it before that even I dread changing anything in it, it's THAT complicated and twisted. I'm amazed it worked in the first place
And if you read my interface thread you'll see that I was tempted at first to change other screens only to find that impossible. Sure, you can add as many draggable items as you want, but show/hide items or even simple buttons are out of our reach in SH4.