I've started using lee neck collet dies for 30.06 and 223 rem and like the results. Lee doesnt make one for the 284 win, so I'm wondering if using a 7-08 die will work? Even though the 284 case is larger dia than the 06, it still fits up into the 06 die.

they will make one for you just cost more...........check their web site

While the body diameter might fit into the die I think the .284 case will be a bit too long from base to neck.

Looking at the Hornady handbook, the 7mm-08 is 1.751" from base to the neck-shoulder junction. The .284 is 1.885", a full .134" longer. The collet fingers for the 7mm-08 are going to come down to that 1.751" length or very close so they would contact the shoulder of the .284 case well before they began closing. The .284 case is also a tad longer overall than the 7mm-08.

The neck lengths themselves are almost identical, .284" for the 7mm-08 and .285" for the .284. You could probably use a washer or spacer under the collet sleeve to fit it. You just need to artificially "lengthen" the collet sleeve by that .134" difference in body length.

OR - again as long as the fatter .284 body will fit inside the collet sleeve, you could get one for a .280 Remington and shorten it appropriately. I'd go this latter route since you just need to modify the die once and done, no need to fuss with washers. A member here shortened a .257 Roberts collet for a .250 Savage AI for me and it worked perfectly.