Depending on what year the truck is, you can do as John suggests. I believe '97 is the year the 150 changed to the current body style, that year and newer there's no provision for factory dual batteries. On earlier models, the second battery tray from diesel versions of the 250 and 350 from the same year will bolt in.
The 250 up to '96 will have the same tray as the diesel versions as well. Then they split the model to the 250 and 250HD for '97. The 250 is a beefed up 150 and again has no factory provision for a second tray, while the 250HD carries the older style and can use the diesel tray again.
There was no '98 250HD, the Super Dutys were introduced mid year as '99 models. With these models you can again use the diesel tray as a bolt in with a few other modifications.
There's another thread currently running on this subject that you might get other ideas from.
Well you'll have to make one up on your own if you really want a second battery.If there is room a fabrication shop should be able to weld one for you that you can bolt in.Might not be worth the hassle though if its real tight.
I seem to remember a company , think it was called timken or something like that , they made all sorts of neat 4x4 stuff , like dual batt trays, They were primarliy jeep , but that was years ago, may have expanded by now.... ?? I'll do a search and see if I can find them and post the link..
Its Tomken mahcine,,, www.tomken.com
But they are still only jeep stuff.. May take a look anyway, they might work for other applications.. Found them under accessories, 2nd or 3rd page.. They said they were desgined for dual optima batts... If youve got a jeep though,, wow, they really have some cool stuff....