Here is an example of taking a 256-bit hex-encoded private key and creating an uncompressed Wallet Input Format (WIF) private key for testnet using the libbitcoin v3.0 bitcoin-explorer (bx) command line interface. bx sub-commands manual pages are located on the right column of its Wiki
% echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff" | bx base58check-encode -v 239
Similarly, here are the results for the associated compressed public key. (Note by convention that "01" is tucked on to the private key.)
echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff01" | bx base58check-encode -v 239
More details for using bx for altcoins can be found at Altcoin Version Mapping. There is a table up top of the wiki that provides insight as to why 239 and 111 are used as versions in bx commands above and below.
Use what ever QR-encoding mechanism you like (e.g., QRencode and encode either 91avAoeY5k21BDnfaHNLv63frRJhn5RmnEw4XgcsbfpWxPg1A9N or cMahgGtgBvsRn7SZFuFCuKGnV8H8yxYLegVcaRsLL2LjD6WFLuAg, but remember the following 256-bit hex encoded private key
is one of the worst private keys ever... but makes the point real clear we have a 256-bit hex-encoded key.
Be aware if the funds are deposited to an uncompressed public address and you provide the private WIF key for a compressed address an import or sweep is not likely to work.
The uncompressed public address for the very simple private key can be found by:
% echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff" | bx ec-to-public -u | bx ec-to-address -v 111
Similarly, the compressed public address can be found by:
% echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff" | bx ec-to-public | bx ec-to-address -v 111