I think Thule makes a truck mounting system but this is definitely way less expensive.
4 U bolts
16 large flat washers
8 large rubber washers
1 foot (or less) of PVC pipe
8 Sealing washers
Bike fork mounts
Bike wheel mounts
A truck with a cap
Saw (hack saw or band saw)
Step 1: Measure
First, it is important to measure the distance between the fork and the back wheel of your bike. There is some variation in this distance from bike to bike and for the trays I have the amount of adjustment is fairly minimal.
Next measure the top of your truck cap as precisely as possible. This is a potentially difficult step because of the curves on the edges of the cap. I measured to the edges of the side windows with a flexible measuring tape to make sure I had a consistent starting point on both sides.
Remember that you want your U bolts on the outside of the trays and fork mounts so that it is harder for someone else to take parts of your rack or your whole bike.
I put the bolts 8.5 inches from each side and 14.5 inches from the ends but you should do measurements of your own.
Step 2: Drill Baby Drill
Drill your holes with a bit that is exactly the same diameter as your bolt. Any extra space is going to make it less likely that the washer seal the hole up correctly.
Also make sure the holes are exactly the right distance apart to fit the U bolt.
Step 3: Cut the Tube
I used a band saw but it might actually be easier to cut them with a hack saw. If you are using a band saw be careful. Cutting round things is dangerous.
I ground a notch in the tubes on one end so that if water does get inside there is an easy way for it to get out that is not through the truck. I do not recommend using an angle grinder for this but it was the most convenient tool I had and it worked.
Step 4: Bolt It Together
Thread your parts onto the U bolts in this order:
Cross bar (plate)
The load bars for the rack should go over the U bolt cross bars.