Crimps are fine, as stated soldering can lead to a cracked wire down the road.

The secret to a quality crimped joint is to use a good crimp tool. What you want to achieve is a "gas tight" cold weld. This is where the pressure is high enough, the wire is fused to the lug.

In a pinch I've slipped a piece of clean copper tube over the wire, pounded the end flat then drilled a hole at the end. Leave the wire end slightly round to act as a lead in for flexing rather than a sudden stop. Tinned copper, as used on most lugs, is better from a corrosion standpoint but this works pretty well.

Have a lug and no crimper? Find an open end wrench that fits the barrel of the lug, clamp the wrench in a vise with the open end up, place the lug in the wrench. If you are using a 3/8" open end, select a 3/8" socket extension to act as a punch and place the square end inline with the wrench and hit with a hammer a time or two. Instant crimp

The wrench constrains the lug from ovaling and extension from escaping ( in one direction ), it also makes for a semi circle on one end. The "punch" forms the cold weld.