I built two just recently. One for my HLT and one for my boiler.
In both cases I have welded 1/2" female connectors installed as close to the bottom of the keggle, just above the curve into the bottom of the keggle.
I then used Watts A-324 1/2" MPT to 5/8" compression fittings. They screw into the connectors on the kegs. From there you would use 1/2" copper pipe. The 1/2" copper tubing fits in the 5/8" compression fitting. The compression fitting provides the ability to adjust the pipe coming out of the fitting. The Pipe can be turned in the compression fitting. This makes it easier to get the end of the dip tube close to the bottom of the keggle without having to build it perfectly.
You could use rigid pipe or soft flexible pipe. It depends on how you want to do it. With the rigid copper pipe you will use a couple of 45 degree fittings or one 90 degree fitting.
For my HLT, I brought the copper pipe out of the compression fitting to about the center of the Keggle and then connected the 90 degree fitting and then added a short piece to bring the dip tube close to the bottom of the keggle. That way I won't have to make adjustments for water remaining in the HLT when doughing in or sparging.
For my boiler I used two 45 degree fittings instead of the 90 degree fitting. I offset these to each other slightly and put a short piece between. This gave me the ability to adjust how close the end of the dip tube is to the bottom of the keggle. I'll use small copper scrubby between the bottom of the keggle and the dip tube to filter out trub.
If you use soft copper tubing you would come out of the compression fitting and bend the tubing to shape it where the dip tube ends close to the bottom of your keggle.
I used the rigid pipe because I could buy a little of it, where the soft tubing I would have needed to buy a substantial coil where I had already built my immersion chiller out of 3/8".