Woohoo! I did it too! Full list of commands on a "fresh" CentOS 6 Docker image:
yum install wget
yum install krb5-devel zlib-devel lksctp-tools-devel util-linux make gcc rpm-build
curl -o openssl-1.0.2k-21.el7_9.src.rpm https://vault.centos.org/7.9.2009/updates/Source/SPackages/openssl-1.0.2k-21.el7_9.src.rpm
rpm -i openssl-1.0.2k-21.el7_9.src.rpm
cd ~/rpmbuild/SOURCES/
sed -i 's/secure_getenv(/getenv(/g' *patch
cd ../SPECS/
sed -i 's/%patch68 -p1 -b .secure-getenv/#%patch68 -p1 -b .secure-getenv/g' openssl.spec
rpmbuild -bb openssl.spec
cd ../RPMS/x86_64
rpm -U openssl-libs-1.0.2k-21.el6.x86_64.rpm openssl-1.0.2k-21.el6.x86_64.rpm
Not sure if openssl-devel-1.0.2k-21.el6.x86_64.rpm would be required, but for regular use I'm guessing it's not.