Jamieson, Fausset and Brown on 1 Chronicles 29:5

5. who then is willing to consecrate his service--Hebrew, "fill his hand"; that is, make an offering (Ex 32:29; Le 8:33; 1Ki 13:33). The meaning is, that whoever would contribute voluntarily, as he had done, would be offering a freewill offering to the Lord. It was a sacrifice which every one of them could make, and in presenting which the offerer himself would be the priest. David, in asking freewill offerings for the temple, imitated the conduct of Moses in reference to the tabernacle (Ex 25:1-8).